Officials getting ready for a massive expansion in the city's prekindergarten enrollment were spending the last day before school starts finishing inspections of every single facility.

Inspectors representing health, fire and other departments were brought in to check the sites, and a team from the Department of Education was expected to complete its walkthroughs of all locations on Wednesday.

Mayor Bill de Blasio's administration announced a snag Tuesday - the shuttering of nine pre-K centers and delaying of start dates at 36 others. Safety and integrity concerns were behind the decision to revoke contracts at the nine centers, which were to serve 265 students. Those students will be reassigned to other centers.

Officials said the vast majority of the 1,700 sites will open on the first day of school Thursday and are poised to serve an unprecedented jump from 20,000 to 53,000 students in a single year.

The expansion - the centerpiece of de Blasio's first year in office - has been closely watched nationally and drew some surprising criticism last week from the city comptroller, who expressed safety concerns about the program.

"Our first rule is that we would only allow sites to open if we can account for health and safety, and there are a number of sites we've worked with over the past few months to make sure they are ready," said Deputy Mayor Richard Buery, the administration's pre-K czar.

"There are, unfortunately, some sites that will not be ready," Buery said in an interview. "If a site doesn't meet those standards, we can't allow them to open at all."

Officials believe most of the 36 sites that won't be ready to open Thursday should be set to open within the next two weeks. Those centers are to educate about 900 students and the missed days will be made up, Buery said.

"Out of 1,700 sites, there are nine that aren't opening, there's less than a half percent," said First Deputy Mayor Anthony Shorris in an interview. "And it's something around 2 percent to be delayed a couple of days. That's pretty good delivery on 53,000 kids and 1,700 sites, to be about 98 percent on the money."

By law, every pre-K staffer must clear a background check, though a teacher whose approval is pending is permitted to deal with children if a cleared supervisor is present. Administration officials stressed that every staffer has submitted a background check but, after nearly a week of inquiries, could not say how many have been cleared.

About 600 public school buildings and 1,100 community-based organizations will host pre-K students. All 45 singled out Tuesday by the de Blasio administration are community-based organizations.

The pre-K plan was de Blasio's signature campaign issue a year ago and helped him vault into City Hall as New York's first Democratic mayor in a generation. Though his plan to finance it with a tax on wealthy New Yorkers died in Albany, he was able to obtain $300 million in state funds for the program, which many feel could act as a model elsewhere in the nation.

De Blasio has embraced the challenge, saying the plan is a "transformative moment" for early childhood education. But the rollout is happening extraordinarily quickly - New York is trying to do in months what smaller cities did in years - and significant failures likely would be catnip to the city's tabloids and could undermine the mayor's future large-scale endeavors.

Comptroller Scott Stringer, normally a reliable de Blasio ally, criticized City Hall last week for being sluggish in sending contracts with pre-K vendors to his office for review. He said that could prevent some necessary safety checks; the mayor downplayed the delays as a "paperwork issue."

There are still about 3,000 pre-k spots still available across the city. Childrre who are 4 are eligible for enrollement. For more information, go to: http://NYC.gov/prek or call 311.